Although glasshouse studies have conclusively demonstrated that S nutrition can affect onion (Allium cepa L.) pungency this has been rarely observed in field-based studies due to difficulties in controlling S nutrition and lack of efficient methods for measurement of flavour bioactives. We have developed a rapid automated method for determination of onion lachrymatory factor ((Z, E)-thiopropanal-S-oxide; LF) based on juice extraction into dichloromethane and gas chromatography (GC) analysis with flame photometric detection. We evaluated this in a field trial of a mild (cv. 'Encore') and a pungent (cv. 'Kojak') onion cultivar grown on a low S soil with and without S addition, under high or low N treatments. No treatments significantly affected bulb fresh weight but S fertilisation significantly increased bulb total S, sulfate, pungency, LF and flavour precursor levels in both varieties. Analysis of bulb flavour precursors by HPLC confirmed that juice LF levels paralleled levels of the flavour precursor S-1-propenyl cysteine sulfoxide. The pungent cultivar also exhibited significant N main effects on bulb LF, total S and sulfate. We also assayed the key S-assimilatory enzyme, APS reductase (APR) in leaves before and during bulbing. Specific activities in the range of 1 to 11 nmol mg −1 ·min −1 were observed in youngest leaves, but only the milder cultivar exhibited significant stage × N × S effects. These findings suggest that sulfur metabolism of mild and pungent onions respond differently to N fertility, and that GC of LF is practical for field-based studies of onion flavour.Abbreviations: ACSO -alkyl cysteine sulfoxide; APR -adenosine 5 phosphosulfate reductase (EC 1.8.99.2); EPY -enzymatically produced pyruvic acid; FID -flame ionisation detector; FPD -flame photometric detector; GC-MS -gas chromatography-mass spectroscopy; HPLC -high performance liquid chromatography; LFlachrymatory factor (Z, E)-thiopropanal-S-oxide; MCSO -S-methyl-L-cysteine sulfoxide; PrCSO -S-1-propenyl cysteine sulfoxide.

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During the 2017-18 growing season, significant outbreaks of leaf blight occurred in Pukekohe, Hawke's Bay and Canterbury commercial onion fields. It was unknown if the causal agent was Stemphylium vesicarium, a pathogen already present in New Zealand that causes stemphylium leaf blight (SLB), or a new introduction of another Stemphylium species. Morphological and molecular characterisation methods were used to identify the pathogen present on diseased onion leaves. The possibility that climate may have been a contributor to the outbreak was evaluated using hourly temperature and relative humidity data, and comparing the 2017-18 growing season with the previous four seasons in these regions when no disease was observed. Our research indicates that the recent leaf blight outbreak in New Zealand was caused by S. vesicarium, and not the introduction of a novel species of Stemphylium. The warm, and wet summer of 2017-18 possibly contributed to the SLB outbreak.

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SummaryThe indigenous Maori people quickly adopted early introductions of potatoes into New Zealand. These early cultivars known as Maori Potatoes are smaller in size and differently coloured when compared to modern potato cultivars and are thought to have remained unselected. The glycoalkaloid content of tubers ranged from 38.7 to 142.6 mg kg -t (m~an 76.5). A taste test conducted on 10 different cultivars showed that they were generally well accepted; however all panellists noted a slightly bitter after taste in some of the cultivars. The texture and flavour were within acceptable limits for all the cultivars tested.